It is highly desirable in numerous communications applications to achieve output power of the order of two to ten watts in the microwave frequency range of 12 to 20 gigaHertz (GHz) from a gallium arsenide (GaAs) field effect transistor (FET). It is believed that these performance requirements are beyond the present state of the art. One of the reasons why such high power has not been attained in the stated frequency range is the low impedance of GaAs FETs, in particular the relatively low shunt impedance of the GaAs active region.
The input and output impedances of GaAs FETs are to be contrasted with the standard 50 ohm output and input impedances of the signal sources and loads which drive and are driven by the GaAs FETs. the impedance of the GaAs FET and the characteristic impedances of the input and output networks connected to the FET cause substantial mismatches. The mismatches are overcome by impedance matching networks that are connected between the input and output networks and a gate (input) electrode of the FET and one of a drain or source (output) electrode of the FET. The matching networks insert substantial losses and reduce the output power of the device. Impedance matching networks also frequently have dimensions such that terminals thereof are more than one wavelength away from an active region of the GaAs FET. Such a situation causes a substantial portion of the available microwave power from the source or amplified by the GaAs FET to be dissipated in a resistive component of the impedance matching networks. Because of these factors, the microwave output power and power gain of the GaAs FET are considerably reduced from the theoretical capability of the GaAs FET.
It is, however, necessary for an impedance match to exist between the GaAs FET and the source and load to which it is connected. Otherwise, a standing wave pattern is developed that causes a substantial loss in microwave power. There are also substantial losses in microwave power because of the finite resistance in transmission lines connecting the GaAs FET device to the source and load. For microwave, high power GaAs FET's, the losses in input and output impedance matching networks connected between the microwave source and load and the active device can be so high as to render the apparent performance of the GaAs FET considerably lower than the intrinsic performance of the GaAs FET.